Diesel and petrol (usually) have a very different fuel delivery system. Petrol often a return system where excess fuel is returned to the tank. So different tank and pipework. Fuel pumps are often different too.
Finding a different engine where the sump fits over the existing steering rack etc may also be a big problem. For example the oil pump may not allow the sump to be modified to clear.
A different gearbox may not fit the transmission tunnel. The chances of the gearlever being in the correct position, remote. And even getting an accurate speedo where the final drive ratio and wheel size is different can be a PITA.
It sounds oh so easy to talk about it over a pint. The reality is very different.
And even if you manage it, getting the vehicle to ride and handle well with a power unit of a different weight may be very tricky indeed.
I've driven a couple of cars with engine replacements that look beautiful at a show, but drive like absolute dogs.
In the UK, diesel cars were pretty rare when lead was removed from petrol. Which was phased in during the 1980s. It was something like a decade on before decent performance diesels became the norm - due to advances in engine managment.
Handling had been OK for a large car of that era. But the extra weight overwhelmed the front damping and the brakes to the point of real fear, I soon learnt to be more careful.
I used to drive a Mk2 Zephyr. We improved the handling quite a bit by fitting radials to the front and crossplies to the rear - before being told 'you can't do that'. ;-) Loved the way it used to pull strongly from near idle in the second gear of three.
I had a Zephyr, I had a rusty Jag. I had no money.
Pinto into Chevette I did in three days start to finish, now that was a quick Q car.
Several V6 Cortinas and Transits over the years, a V8 CF Bedford was fun, but the best fun was the Lotus Escort vans that several of us had, Friday night pub night was always more exciting on the way home.
Most amusing car I drove was one I built for my ex-wife. It was an early Mini Traveller Woody, and I fitted an MG 1100 engine, and Cooper 'S' gearing. Apart from slim wheel spacers, and a rev-counter, it was indistinguishable from standard. It turned a car that could just about scrape 78mph into one that could get very close to a ton.
I enjoyed driving it almost as much as my proper 'S'!
Not sure where you got that figure from, but there are plenty of internet sources that state that the 998 could nudge over 90 mph.
Didn't take much to get one over that though. Larger SU and LCB manifold would just about do it.
The MG 1100 engine I used had a stage one head and a larger carb.
With 'S' gearing, 6,000 rpm equated to 97 mph. I had on occasion to lift off to avoid over-revving in top due to the propensity of the 1100 engine to break its long-stroke crankshaft.
My 'S' was a 1,293cc one with a half-race engine. It had just about everything done to it. Brake tested at the wheels, it made 100 bhp.
On standard 'S' gearing (3.4:1), peak revs equated to 124 mph. For rally use, I used the 1,071 ratio of 3.7:1, and had to watch the rev-counter carefully on Motorway trips home from events.
The speedo fitted to the Traveller was no indicator of its speed however!
From memory, it peaked at 85 mph, and wouldn't have read accurately due to the change in final drive ratio. The needle just used to bounce around off the stop at anywhere near top speed; I was surprised it survived.
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